1. Field of the Invention
The present invention is directed broadly to micromachined memory devices and more particularly to techniques for ultrahigh density writing with a micromachined probe on an erasable magnetic medium.
2. Description of the Background
The invention of various scanning probe microscopy technologies has made possible the development of ultrahigh density data storage technologies based upon them. The use of an atomic force microscope for recording and playing back topographical features in a plastic substrate has been demonstrated. Such scanning probe storage technologies make possible exceedingly high storage densities in the 10.sup.10 to 10.sup.12 bits/in.sup.2 range. Most of the proposed methods of storage which have been investigated using those techniques utilize write-once technologies in which it is possible to write-once on the storage medium and read it back repeatedly. Whereas such write-once technology does have applications, it is desirable to have storage technologies which provide the capability to write, erase, and rewrite information repeatedly.
Magnetic storage media such as those used in magnetic and magneto-optic disk and tape drives are known to be writable, erasable, and rewritable using magnetically switchable magnetic recording heads. However, when the head dimensions become small compared to the distance over which exchange energy tightly couples the magnetization, the magnetization in the head assumes a single domain state and becomes very difficult to switch. In such circumstance, to switch the magnetization in the head, it becomes necessary to overcome the shape anisotropy of the head. A long wire-like probe head would, for example, require a field equal to 0.5 M.sub.s to switch its magnetization. For a typical soft magnetic material such as Permalloy, a field of 400 KA/m is required, which is much too large to generate with a coil wound around the probe. Because switching the magnetization of a ferromagnetic probe tip directly by an applied magnetic field generated by a current does not appear to be viable, other means of switching must be developed.
One writing technique which has already been demonstrated to work at densities approaching 100 Gbit/in.sup.2 is to use heat generated by laser light coupled into an optical fiber, which has been tapered to a very small aperture at the end, to raise the temperature of a small region of a magnetic thin film above its Curie temperature in the presence of a magnetic field. R. E. Betzig, et al., "Near-Field Magneto Optic and High Density Data Storage", Applied Physics Letters, Vol. 61(2), p. 142-144 (1992). When the film cools down after the laser energy is removed, its magnetization assumes the direction of the externally applied magnetic field. That thermomagnetic recording technique was demonstrated through joint work of researchers at AT&T Bell Laboratories and Carnegie Mellon University to be capable of recording 60 nm diameter domains on 120 nm diameter center-to-center spacing, or a density of about 45 Gbit/in.sup.2. Higher densities might have been achievable, but the roughness of the film prevented the researchers from bringing the tip closer to the medium.
Another technique is to use magnetic resonance phenomena to switch the magnetization of either a selected bit in the medium or of the probe head itself. Artman and Charap studied the use of ferromagnetic resonance to nucleate domains in garnet materials several years ago at Carnegie Mellon University. J. O. Artman, et al., "Microwave Generation of Bubble Domains in Magnetic Thin Films". IEEE Transactions on Magnetics, Vol. Mag 19, No. 5, p. 1814-1816 (1983); J. L. Dong, et al., "The Investigation of the Process of Microwave Bubble Generation in a Bubble Material", J. Appl. Phys. 63(9), p. 4609-4615 (1988). They showed that by applying a magnetic field to a magnetic material and driving the material sufficiently hard with a radio frequency field tuned to the resonant frequency at which the spins precessed, the precession could be made sufficiently large in amplitude to cause the magnetization in the material to switch from one orientation to another. To write using a magnetic probe head using that technique, one can apply a radio frequency field to a magnetic medium at a frequency which is close to the resonant frequency of the medium and use the localized field generated by a magnetic probe head to tune the resonant frequency of a selected bit location to that of the applied radio frequency field, causing it to switch magnetization orientations. Despite that work, the need still exists for techniques that allow for the ultrahigh density writing with a probe on an erasable magnetic medium which are easy to implement with existing technologies, reliable, and fast.